The present invention relates to an improved process for producing acetal copolymers and, in particular, to such a process which enhances the thermal stability of acetal copolymer resins.
Acetal copolymer resins are characterized by a predominately carbon-oxygen oxymethylene backbone occasionally interrupted by a comonomer unit having a carbon-carbon linkage. The simple linear chain structure and the relative shortness of the carbon-oxygen bond impart properties of high crystallinity and relatively high density to acetal resins in the solid state. Commercial products of acetal resins have high stiffness, good dimensional stability, high tensile and impact strength, good abrasion resistance, and a low coefficient of friction, all of which make acetal resins excellently suited for, among other things, replacement of metals and provision of fatigue-resistant rigid articles and/or parts. Typically, such items are manufactured as shaped articles by injection molding or extrusion processes.
The copolymers forming acetal resins, however, contain monomeric units which have comparatively unequal stability to degradation. Furthermore, resistance of the copolymer to degradation depends, in many cases, as much upon the relative position of the monomeric units within the polymeric chain as it does upon the inherent monomeric structure. For example, if a copolymer is susceptible to degradation by a mechanism which attacks the ends of the polymeric chain, it can be seen that if the ends of the molecules are susceptible to degradation, the copolymer will have less stability than one in which the molecule ends are relatively stable.
Acetal copolymers, which, as has been known for years, may be prepared, for instance, by the polymerization of trioxane--a cyclic trimer of formaldehyde--and ethylene oxide, have recurring oxymethylene groups, --OCH.sub.2 --, directly attached to each other as well as oxyethylene groups, --OCH.sub.2 CH.sub.2 O--, interspersed throughout the polymeric chain. Oxymethylene copolymers of this type are described in commonly assigned U.S. Pat. No. 3,027,352 to Walling et al., the pertinent parts of which are incorporated herein by reference. Such copolymers contain polymeric chains having 60 to 99.6 percent oxymethylene units some of which form the end groups of such chains. These copolymers are subject to physical degradation under high temperture conditions due to the decomposition of formaldehyde units having a terminal hydroxyl group, i.e. CH.sub.2 O.sub.n H. Accordingly, even though acetal resins possess many excellent physical properties, they are subject to a certain degree of degradation, particularly under the influence of heat, unless they have been subjected to treatment which effectively eliminates the relatively unstable characteristics inherent therein.
It has been discovered that thermal degradation of acetal resins resulting from the splitting off of successive formaldehyde units, commonly referred to as a "zipper" reaction, is halted when the reaction reaches a comonomer on the polymeric chain having a comparatively stable structure, such as the carbon-to-carbon bond present in oxyethylene. To that end, efforts over the years to improve the thermal stability of acetal copolymers have been directed toward eliminating the terminal polyoxymethylene groups as completely as possible. The primary method developed for removing the oxymethylene end groups is by hydrolysis, which has the effect of shifting the hydrogen atom of the terminal hydroxyl group to the oxygen atom of the next adjacent oxymethylene group, while simultaneously detaching the endmost oxymethylene group from the polymeric chain.
Exploration into the chemistry and technology necessary to sustain efficient production level hydrolysis of polyoxymethylene copolymers has led to several further developments in the art of commercial acetal resin production. For example, U.S. Pat. No. 3,174,948 to Westfield et al., which is assigned to the same assignee as the present application, describes an aqueous alkanol solvent (for copolymers) which allows for complete solution of the copolymer at lower temperatures than either water or the alkanol alone. In commonly assigned U.S. Pat. No. 3,219,623 to Berardinelli a process for stabilizing normally solid oxymethylene copolymers is described which includes a hydrolysis reaction of the copolymer under non-acidic, and preferably alkaline, conditions with a hydroxy-containing material such as water or alcohol.
U.S. Pat. No. 3,301,821 to Asmus et al. discloses a process for thermostabilizing copolymers having terminal oxymethylene groups by splitting off the terminal oxymethylene groups when the copolymer is treated at a temperature of 100.degree. C. to 160.degree. C. at autogenous pressure with a saturated vapor mixture of water, a volatile organic swelling agent, and a volatile base for catalyzing the removal of oxymethylene groups. The copolymer is suspended in a wire mesh basket within an autoclave wherein the reactive vapor atmosphere is generated. In order to prevent discoloration which may occur, it is suitable to add urea to the liquid phase in an amount of 0.5 to 5% based on the weight of the liquid phase.
Celanese-assigned U.S. Pat. No. 3,318,848 to Clarke describes a melt hydrolysis process which involves a mixture of the copolymer and a reactant selected from the group consisting of water, alcohols, and mixtures thereof. The polymer melt is hydrolyzed in a single reaction zone at a temperature of from 160.degree. C. to 240.degree. C. and a pressure in the range of from 150 to 10,000 psia. U.S. Pat. No. 3,418,280 to Orgen, which is also assigned to Celanese, shows an improvement in the melt hydrolysis processes described in the Clarke patent which improvement includes reacting the polymer with the reactants at a pH between 9.5 and 11.0.
In both processes the polymer-reactant system is a single phase system reacted in a reaction zone which is usually a single screw extruder. Such a reactor has a very limited degree of mixing and can handle only a low level of hydrolysis solvent. Furthermore, the devolatization capacity is very limited.
A further method of hydrolysis described in U.S. Pat. No. 3,419,529 to Chase et al.; U.S. Pat. No. 3,428,605 to Smith et al.; and U.S. Pat. No. 3,505,292 to Smith et al., all of which are assigned to the same assignee as the present application, is accomplished by forming a slurry of solid polymer particles in a liquid hydrolysis reaction medium while the slurry is transported to a further stage of polymer treatment without substantial backmixing. The hydrolysis medium which is ideally composed of water, a water-soluble non-acidic organic compound having an oxygen atom directly bonded to a carbon atom, and, in accordance with U.S. Pat. No. 3,428,605 to Smith et al., between 50 and 99 weight percent of trioxane, is maintained at a temperature causing the polymer to swell; but not to dissolve, become tacky or agglomerate in the hydrolysis medium. A suitable type of equipment used to continuously convey the slurry while carrying out the chemical hydrolysis reaction in this process is a non-backmixing screw conveyor.
More recently, Celanese-assigned U.S. Pat. Nos. 3,839,267 and 3,853,806 to Golder describe a heterogenous melt hydrolysis process for stabilizing copolymers containing oxymethylene end groups in which the polymer is reacted while in the molten state whereas the reactant mixture is in the gaseous or vapor state--thus giving rise to the term heterogeneous. As in the previous hydrolysis process, the hydrolysis reaction is usually intended to be carried out in a single screw extruder.
While the hydrolysis of acetal copolymers increases the thermal stability of said copolymers, a side product of the principal reaction, formaldehyde, simultaneously becomes immediately available as a reactant for several other reactions within the reaction zone which adversely effect the end product. For example, traces of oxygen can oxidize formaldehyde, especially at elevated temperatures, to acidic species like formic acid, performic acid, etc.
Furthermore, the base-generating catalyst triethylamine, TEA, which is commonly used in hydrolysis reactions, can react (1) with formic acid to form triethylammonium formate: ##STR1## and (2) with traces of HF (which may be present as a result of using BF.sub.3 as a polymerization catalyst) to form triethylammonium fluoride: EQU (C.sub.2 H.sub.5).sub.3 N+HF=(C.sub.2 H.sub.5).sub.3 NH.sup.61 F.sup..crclbar.
Both triethylammonium formate and triethylammonium fluoride have been demonstrated to induce degradation of acetal copolymers.
Formaldehyde may also undergo another reaction under alkaline conditions, namely aldol-type condensation to form formose sugars according to the equation below: ##STR2## which eventually lead to undesirable formation of color in the acetal copolymer product.
Thus, in addition to the desired hydrolysis reaction, a number of undesirable side reactions can simultaneously occur inside the reactor, which, because of the very nature of such reactions, can adversely affect the final properties of the copolymers. It is clear, therefore, that an efficient removal of formaldehyde TEA and other undesirable species (e.g., formic acid, HF, etc.) which might be present during hydrolysis should improve both the thermal stability and other qualities of the copolymer. In the case of solution type hydrolysis, many of the harmful products may be disposed of by a washing treatment, but since melt-hydrolyzed copolymer does not undergo a thorough washing treatment, some of the harmful products are retained in melt-hydrolysis copolymer thereby causing an inferior quality product. This is an unfortunate problem in the area of commerical acetal resin production because the melt hydrolysis method of producing oxymethylene copolymers consumes as much as one third less energy than the solution type hydrolysis.
It is therefore an object of the present invention to overcome the problems, such as those itemized above, associated with undesirable side products and side reactions resultant therefrom during melt-hydrolysis of acetal copolymers.
Furthermore, the limitations of the Clarke process and apparatus as set forth above are obviated thereby allowing a more efficent hydrolysis process, which, in turn, reduces the residence time of the copolymer in the hydrolysis reactor. This adds to the quality of the resultant polymer, by, for one thing, reducing undesirable coloration of the resin product.